Tilted Grating Sensor

ABSTRACT

The present invention relates to a sensor using a tilted fiber grating to detect physical manifestations occurring in a medium. Such physical manifestations induce measurable changes in the optical property of the tilted fiber grating. The sensor comprises a sensing surface which is to be exposed to the medium, an optical pathway and a tilted grating in the optical pathway. The grating is responsive to electromagnetic radiation propagating in the optical pathway to generate a response conveying information on the physical manifestation.

FIELD OF THE INVENTION

The invention relates to a sensor using a tilted fiber grating to detectexternal events that induce measurable changes in an optical property ofthe tilted fiber grating. The sensor can be used to sense a wide rangeof biological processes, biological elements, chemical elements ormanifestations of physical phenomena such as temperature, strain orindex of refraction.

BACKGROUND OF THE INVENTION

Fiber Bragg grating (FBG) sensors have a wide range of applications suchas pressure-strain sensors, temperature sensors, micro-bending sensorsand external refractive index sensors. As these optical sensors areinherently immune from electromagnetic interference and chemicallyinert, they are very attractive in bio-chemical applications andhazardous surroundings.

The sensing mechanism most often used in FBGs arises from the fact thatthe reflection wavelength for the forward propagating core mode varieslinearly with temperature and strain. Since the wavelength can bemeasured with an accuracy of 10 pm relatively easily near 1550 nm, thisrepresents a relative resolution of about 6 ppm. A variant of the sameconcept uses so-called Long Period Gratings (LPG) where coupling occursbetween the forward propagating core mode and forward propagatingcladding modes. In this case, the sensitivity of the resonancewavelength to perturbations can be greatly enhanced for some of thecladding modes. Furthermore, since LPGs involve cladding modes there hasbeen great interest in using these for refractive index sensing byimmersing the fibers in the medium to be measured. Special absorbingcoatings can also be used to detect chemicals or liquids through therefractive index changes (or volume changes) induced in the coatings.However, the spectral response of LPG resonances is rather broad (widthgreater than 10 nm) making high accuracy measurements of smallwavelength changes more difficult than with FBGs. Ideally, suchrefractive index sensors should be able to distinguish different kindsof perturbations, and insensitivity to temperature is often particularlydesirable. A problem with both FBG and LPG sensors is that they areintrinsically quite sensitive to temperature, with resonance wavelengthsdrifting by about 10 pmfC, unless special bulky packaging is used toathermalize the device. In order to circumvent this problem inrefractive index sensors, devices proposed so far have involvedcombination of gratings in one sensor such as two different types offiber Bragg gratings, two fiber Bragg gratings with different claddingdiameters and a long period grating (LPG) with a Bragg gratings. In suchcases, the differential sensitivity of the two gratings to temperatureand the desired measurand is used to discriminate between the twoperturbations.

Against this background it can be clearly seen that the current sensortechnology has drawbacks. It is therefore the aim of the presentinvention to alleviate those drawbacks.

SUMMARY OF THE INVENTION

As embodied and broadly described herein the invention provides a sensorfor sensing at least one physical manifestation occurring in a medium.The sensor comprises a sensing surface for exposure to the medium, anoptical pathway and a tilted grating in the optical pathway. The gratingis responsive to electromagnetic radiation propagating in the opticalpathway to generate a response conveying information on the at least onephysical manifestation.

As embodied and broadly described herein the invention also provides asensor for sensing a physical manifestation occurring externally of thesensor. The sensor comprises a sensing surface for exposure to thephysical manifestation, an optical pathway and a tilted grating in theoptical pathway. The tilted grating is responsive to electromagneticradiation propagating in the optical pathway to induce SPR adjacent thesensing surface.

As embodied and broadly described herein the invention also provides amethod for detecting the presence of bacteria in a medium. The methodcomprises providing a sensor having a sensing surface, the sensor alsohaving an optical pathway containing a tilted grating, placing thesensing surface in contact with the medium and determining from aresponse of the sensor if the bacteria is present in the medium.

As embodied and broadly described herein the invention also provides amethod for detecting the presence of virus in a medium. The methodcomprises providing a sensor having a sensing surface, the sensor havingan optical pathway containing a tilted grating, placing the sensingsurface in contact with the medium and determining from a response ofthe sensor if the virus is present in the medium.

As embodied and broadly described herein the invention also provides amethod for measuring the concentration of sugar in a medium. The methodcomprises providing a sensor having a sensing surface, the sensor havingan optical pathway containing a tilted grating, placing the sensingsurface in contact with the medium and determining from a response ofthe sensor the concentration of sugar in the medium.

As embodied and broadly described herein the invention also provides amethod for measuring the concentration of alcohol in a medium. Themethod comprises providing a sensor having a sensing surface, the sensorhaving an optical pathway containing a tilted grating, placing thesensing surface in contact with the medium and determining from aresponse of the sensor the concentration of alcohol in the medium.

As embodied and broadly described herein the invention also provides amethod for detecting the presence of a chemical or biological element ina medium. The method comprises providing a sensor having a sensingsurface, the sensor having an optical pathway containing a tiltedgrating, placing the sensing surface in contact with the medium anddetermining from a response of the sensor if the chemical or biologicalelement is present in the medium.

As embodied and broadly described herein the invention also provides amethod for measuring a degree of curing of a curable material. Themethod comprises the steps of providing a sensor having a sensingsurface, the sensor having an optical pathway containing a tiltedgrating, placing the sensing surface in contact with the curablematerial determining from a response of the sensor the degree of curingof the curable material.

As embodied and broadly described herein the invention also provides anelongation strain sensor. The elongation strain sensor comprises anoptical pathway and a tilted grating in the optical pathway to generatea response conveying information on elongation strain acting on thesensor.

As embodied and broadly described herein the invention also provides amethod for measuring elongation strain. The method comprises receiving aresponse from a sensor containing a tilted grating subjected toelongation strain, the response conveying information on reaction of thetilted grating to elongation strain and on reaction of the tiltedgrating to temperature, processing the response of the tilted grating todistinguish the reaction of the tilted grating to elongation strain fromthe reaction of the tilted grating to temperature.

As embodied and broadly described herein the invention also provides anapparatus for measuring elongation strain. The apparatus comprises anelongation strain sensor having an optical pathway, a tilted grating inthe optical pathway to generate a response conveying information onelongation strain and temperature acting on said sensor and a signalprocessing unit to process the response of the tilted grating anddistinguish in the response to reaction of the tilted grating toelongation strain from the reaction of the tilted grating totemperature.

As embodied and broadly described herein the invention also provides abending strain sensor. The bending strain sensor comprises an opticalpathway and a tilted grating in the optical pathway to generate aresponse conveying information on bending strain acting on the sensor.

As embodied and broadly described herein the invention also provides amethod for measuring bending strain. The method comprises receiving aresponse from a sensor containing a tilted grating subjected to bendingstrain, the response conveying information on: reaction of the tiltedgrating to bending strain; reaction of the tilted grating totemperature. The method also comprises processing the response of thetilted grating to distinguish the reaction of the tilted grating tobending strain from the reaction of the tilted grating to temperature.

As embodied and broadly described herein the invention also provides anapparatus for measuring bending strain. The apparatus comprises abending strain sensor, having an optical pathway, a tilted grating insaid optical pathway to generate a response conveying information onbending strain and temperature acting on said tilted grating, a signalprocessing unit to process the response of the tilted grating anddistinguish in the response to reaction of said tilted grating tobending strain from the reaction of the tilted grating to temperature.

As embodied and broadly described herein the invention also provides apressure sensor. The pressure sensor comprises an optical pathway, atilted grating in the optical pathway to generate a response conveyinginformation on bending strain acting on said sensor, a flexible member,the optical pathway being mounted to the flexible member. The flexiblemember flexes in response to pressurized fluid and induces in the tiltedgrating bending strain.

As embodied and broadly described herein the invention also provides asensor, for sensing at least one physical manifestation occurring in amedium. The sensor comprises an optical pathway an interface coupledwith the optical pathway, the interface being responsive to the physicalmanifestation to induce strain on said optical pathway; a tilted gratingin the optical pathway, the tilted grating being responsiveelectromagnetic radiation propagating in the optical pathway to generatea response conveying information on the strain induced on the opticalpathway.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of examples of implementation of the presentinvention is provided hereinbelow with reference to the followingdrawings, in which:

FIG. 1 a is a high level schematic of a TFBG sensor;

FIG. 1 b is a graph showing the simulated transmission spectrum of aTFBG;

FIG. 2 a is a graph showing the differential wavelength shift forselected TFBG resonances as a function of the refractive index of thesurrounding medium for a 125 μm cladding diameter;

FIG. 2 b is a graph showing the differential wavelength shift forselected TFBG resonances as a function of the refractive index of thesurrounding medium for an 80 μm cladding diameter;

FIG. 3 is a graph showing the experimental TFBG transmission spectrumfor three values of SRI;

FIG. 4 is a graph showing the temperature sensitivity of several TFBGresonances and differential shift from the core mode resonance;

FIG. 5 is a graph showing the effect of bending of a TFBG on itstransmission spectrum;

FIG. 6 is a scheme for interrogating an individual cladding moderesonance using a photo-detector collecting all the reflected light. Theinsets in the figure indicate the transmission spectrum of a typicalTFBG (near a cladding mode resonance) and the reflection spectrum of theinterrogating Fiber Bragg Grating (FBG). The position of the FBGreflection peak determines which cladding mode is interrogated.

FIG. 7 is a graph showing the displacement of the reflected power from aTFBG-FBG pair when the FBG is tension-tuned across a TFBG cladding moderesonance.

FIG. 8 is a graph showing the transmission spectrum of a TFBG;

FIG. 9 is a cross-sectional view of optical fiber containing a TFBG thatproduces surface plasmon resonances;

FIG. 10 shows a dielectric/conductor interface on an optical fiber toillustrate the Kretchman configuration for exciting SPR.

FIG. 11 is a graph illustrating the transmission spectrum of the samegrating as the one of FIG. 8, but with a 20 nm gold layer in air.

FIG. 12 is a graph showing the transmission spectrum of gold-plated TFBGin a sugar solution. The bracket identifies the peak position of theanomalous resonance.

FIG. 13 is a graph showing the SPR peak wavelength (λ_(p))andcorresponding cladding mode effective index on the refractive index ofthe external medium at 589 nm (n_(D))

FIG. 14 is a block diagram of a measuring apparatus using a tiltedgrating, according to an example of implementation of the invention.

FIG. 15 is a block diagram of the signal processing device of themeasurement apparatus shown in FIG. 14.

FIG. 16 is a scheme of a diaphragm 2000 that is made of flexiblematerial and exposed to pressure on its side 2002. A sensor 2004 ismounted to the diaphragm. The sensor is placed on the convex side of thediaphragm 2000.

FIG. 17 is a schematic representation of self-assembled monolayersmolecules on the sensing surface. The self-assembled monolayersmolecules comprises a recognition analyte which associates with ananalyte of interest.

FIG. 18 is a block diagram of a measuring apparatus using a tiltedgrating, according to an example of implementation of the invention. Theapparatus comprises a reflector as well as an optical coupler.

In the drawings, embodiments of the invention are illustrated by way ofexample. It is to be expressly understood that the description anddrawings are only for purposes of illustration and as an aid tounderstanding, and are not intended to be a definition of the limits ofthe invention.

DETAILED DESCRIPTION

A non-limiting example of implementation of this invention uses anoptical structure in which light is guided by a core medium able tosupport one or few guided modes, surrounded by a finite-sized claddingmedium whereas the cladding itself acts as a multimode waveguide.Specific examples of this structure include optical fibers and planarlight circuit (PLC). In a weakly tilted fiber Bragg grating (TFBG)sensor both a core mode resonance and several cladding mode resonancesappear simultaneously, as shown in FIG. 1. Weakly tilted gratings aredefined as having a tilt angle greater than zero but less than 45degrees relative to the propagation axis, so that there is a non-zerocore mode reflection induced by the grating. In a specific example, thetilt angle is in the range from greater than zero and about 20 degrees.In a more specific example the tilt angle is in the range from about 2degrees to 12 degrees. This has several advantages. The cladding moderesonances are sensitive to the external environment (refractive index,deposited layer thicknesses, etc.) and to physical changes in the wholefiber cross-section (shear strains arising from bending for instance),while the core mode (Bragg) resonance is only sensitive to axial strainand temperature. The temperature dependence of cladding modes is similarto that of the core modes, so that the effect of temperature can beremoved from the cladding mode resonance by monitoring the wavelengthdifference between the core mode resonance and selected cladding moderesonances. Using this technique, sensors can be made for sensingphysical manifestations such as elongation strain, bending, or measuringthe Surrounding Refractive Index (SRI), that aretemperature-independent.

Another feature which may constitute an advantage of the TFBG over theLPG to couple to cladding modes is that the resonances are as narrow asthose of a FBG, i.e. of the order of a few hundred pm, instead ofseveral tens of nm for LPGs. Therefore, wavelength interrogation forTFBG occurs over narrow wavelength ranges and makes it possible to usecommonly available light sources, detectors, couplers and multiplexers.In particular, a typical entire TFBG spectrum fits easily within thestandard telecommunication bands (1530-1560 nm). Also, by using theresonance positions and/or strengths individually, allows extractingmultiple sensing parameters from a single sensor. More specifically, amulti-functional sensor can be built by monitoring several cladding moderesonances (or groups of resonances) which have different sensitivitiesto different physical manifestations. For instance, bending affectsmainly low order cladding modes, while SRI variations affect higherorder cladding modes preferentially, and neither perturbation affectsthe core mode resonance.

As is well known, the Bragg reflection and cladding mode resonancewavelengths λB and λiclad of TFBG are determined by a phase-matchingcondition and can be expressed as follows:

λ_(B)=2n _(eff) A/cosθ(1)

λ^(i) _(clad)=(n ^(i) _(eff) +n ^(i) _(clad))A/cos θ  (2)

where neff, nieff and niclad are the effective indices of the core modeat λB and the core mode and the ith cladding mode at λicladrespectively, and λ and θ are the period and the internal tilt angle ofthe TFBG. The tilt angle is defined as the angle formed by the TFBG andthe imaginary axis of the optical pathway containing the TFBG alongwhich the optical signal interrogating the TFBG propagates. When theoptical pathway is defined by an optical fiber, the axis of the opticalfiber will usually constitute the axis of optical signal propagation.For TFBG structures, if only the Bragg and cladding mode wavelengthshifts (ΔλB, Δλiclad) caused by external refractive index changes(Δnext) and temperature changes (ΔT) are taken into account, thewavelength shifts ΔλB and Δλiclad can be written from equations (1) and(2) as follows:

In standard optical fibers, the effective index of core modes (n_(eff),n^(i) _(eff)) are insensitive to the external refractive index changes,so the equations (3) and (4) may be simplified as following:

Equations (5) and (6) show that the cladding resonances will change withthe external refractive index, but not the Bragg wavelength. It can alsobe seen that the different cladding modes may have differentsensitivities to SRI changes. The differential wavelength shift, whichcan be used as a sensing quantity, is given by Equation (7). The secondterm on the right-hand-side of Equation (7) represents the temperaturedependence of the relative wavelength shift. Using the thermal expansioncoefficient of silica (0.55×10⁻⁶/° C.) and an extreme worst caseestimate of 1×10⁻⁶/° C. for the difference in the temperature dependenceof the refractive indices of the core and cladding glasses (which changeindividually by about 10×10⁻⁶/° C.)), it can be shown that thetemperature dependence of the differential resonance is less than 0.54pm/° C. for the differential wavelength shift.

Accordingly, the value of Δn_(ext) determined from Equation (7) is thevalue for the SRI at the sensing temperature (and the temperature can bedetermined independently by the core mode resonance value from Equation(5)).

A number of simulations of TFBG constructions will now be discussed toillustrate the main features of the TFBG sensor. Those simulations havebeen made using a commercial optical fiber grating software simulator,such as OptiGrating version 4.2 from Optiwave Corp.,(http://www.optiwave.com).

First, a TFBG with grating period of 534 nm and internal tilt angle of4.5° was simulated in a CORNING SMF-28 standard optical fiber as afunction of the refractive index of the external medium (SRI). Thestructure of such TFBG grating is shown at FIG. 1 a. Broadly speaking,the TFBG is integrated into an optical pathway 10 which is in the formof an optical fiber. The optical pathway has a core 12 surrounded by acladding 14. The TFBG grating 16 is written into the core 12. The axisof the TFBG 16, which is perpendicular to the bars forming the grating16 is at an angle of 4.5° with relation of the axis 18 of the opticalpathway 12, along which an optical signal interrogating the TFBG 16propagates. When the TFBG 16 is interrogated by an optical signal theTFBG 16 produces a response that has two components. One of thecomponents is the core mode resonance 20 which is reflected back towardthe source of the optical signal. The other is the cladding moderesonance 22 which includes a series of individual emissions atdifferent wavelengths that propagate in the cladding 14 toward the outersurface of the optical pathway.

FIG. 2( a) shows the wavelength shift of 5 different cladding moderesonances (relative to the core mode resonance, which remains fixed)when the SRI changes from 1.33 to 1.44. The highest order cladding modesbegin to shift sooner as the SRI increases because their mode fieldintensity extends further into the external medium as they are closer totheir cut-off. High order modes sequentially disappear (from the shortwavelength side, as shown in FIG. 1 b) when the SRI reaches values forwhich they are cut-off.

Note that the final slope before cut-off for the various resonancesappears to converge for all the modes to a common value near 180 pm/%SRI. Therefore, monitoring several resonances simultaneously allows highsensitivity measurements of SRI over a larger range of values.

Higher sensitivity also can be achieved by reducing the cladding layerthickness (by etching in diluted hydrofluoric acid for instance), or byusing fibers that are fabricated with smaller diameter claddings: thisresults in fewer cladding modes that are more widely separated inwavelength. If the cladding layer diameter is reduced from 125 μm to 80μm, the SRI sensitivity becomes 350 pm/% SRI, as shown in FIG. 2( b). Byfurther reducing the cladding layer diameter, much higher wavelengthshifts can be expected. Note that a potential problem may arise if thecladding layer diameter is reduced to less than 30 μm; at or near thisvalue the Bragg wavelength becomes sensitive to the SRI changes and thein-fiber temperature reference may be lost. Another option is to usefunctional thin films to “pull” certain modes out of the fiber claddingand make them sensitive to specific environments.

The results of experimental work conducted for SRI sensing andtemperature insensitivity are shown in FIGS. 3 and 4 respectively. TheTFBG is written in a CORNING SMF 28 fiber using ArF excimer laser lightat 193 nm and a phase mask to generate the grating pattern. This is notto be considered a restriction since any method of fabrication for FBGsis applicable to the sensors described here. The tilt angle was adjustedexperimentally until the core and main cladding mode resonancetransmission dips achieved comparable attenuation levels. The exactvalue of the tilt angle is not a critical parameter as it onlyinfluences the relative strengths of the core and cladding moderesonances for a given sensor (and hence only impacts the details of thesensor interrogation scheme chosen in a particular case). The SRIsensitivity was obtained by measuring the transmission spectra of theTFBG immersed in calibrated refractive index fluids (from CargilleLaboratories). FIG. 3 shows the changes in transmission of the highestorder cladding modes with SRI of 1.40, 1.42 and 1.44, illustrating thatresonance shifts increase with mode order and that large changes occurwhen modes approach cut-off.

The measured wavelength shifts of modes approaching cut-off are 0.275 nmand 0.218 nm when the SRI changes from 1.40 to 1.42 and 1.42 to 1.44respectively. The latter shift corresponds to a sensitivity of 300 pm/%SRI, actually higher than the theoretical values indicated previously.If the power level within a narrow band at a fixed wavelength ismonitored instead of the wavelength itself, the above mentionedwavelength sensitivity translates into a change in transmitted power of20% for a change in SRI of 0.7% since resonances are at most 200 pm widefor 1 cm-long gratings, and several resonances have an amplitude ofabout 1 dB. By monitoring simultaneously the power level near the coremode resonance and assuming that 0.1% level difference between the twomeasurements can be detected, the minimum detectable SRI is of the orderof 4×10⁻⁵. It is important to note that the first resonance on the shortwavelength side of the core mode Bragg reflection is a ghost mode whichdoes not appear in simulations unless a UV induced break in symmetry orrelatively large core index increase is included in the calculation.This ghost mode may be useful in detection of fiber bends and shearstresses.

As mentioned above, it is desirable that the wavelength shifts measuredin SRI sensing do not depend on the temperature of the sensor. Theexperiments show that while individual resonances of core and selectedcladding modes each drift by ˜10 pm/° C. (for the first resonance (LP₀₁:Bragg wavelength resonance), the second resonance (ghost mode resonance)and the seventeenth resonance), as shown in FIG. 4, the relativewavelength shift between the cladding modes and the Bragg wavelength isless than 0.4 pm/° C. This represents a ˜27 times reduction insensitivity from the resonance wavelengths themselves.

This confirms the usefulness of the self-referencing feature for makingthis SRI sensor temperature-independent.

Finally, the experimental results show that the TFBG spectrum changes asa function of bending in FIG. 5 (using a stronger grating written inhydrogen-loaded fiber). As expected since the fiber core lies on aneutral strain axis for pure bending, the core mode resonance and highorder cladding modes remain unchanged but the first few low ordercladding mode resonances change by several dB for the bending radiitested. Therefore, a single sensor can be used to detect three differentphysical manifestations simply by interrogating it over three differentwavelength ranges: 1) near the Bragg resonance for temperature; 2) nearlow order cladding modes for bending; 3) at the short wavelength end ofthe transmission spectrum for SRI.

A particularly simple interrogation scheme for the TFBG could beconstructed by spectrally slicing the transmitted light using an arrayedwaveguide grating (AWG) butt-coupled to a detector array. The spacingand widths of the resonances are quite compatible with standard 40 or 80channels AWGs that are widely available from telecommunicationscomponent vendors. Instead of tracking wavelength shifts, the detectorsmonitor power level changes at fixed wavelength positions. One channelof the AWG interrogator can be used in closed loop to thermally tune theAWG (forcing its wavelength comb to follow the core mode resonance)while the output of another channel would reveal the power level changesassociated with the relative wavelength shift of a cladding moderesonance. Level discretization and digital processing can then be usedto extract meaningful data from the TFBG response. For more informationon this interrogation scheme, the reader is invited to refer to thearticle of G. Z. Xiao, P. Zhao, F. G. Sun, Z. G. Lu, Z. Zhang, and C. P.Grover, “Interrogating fiber Bragg grating sensors by thermally scanninga demultiplexer based on arrayed waveguide gratings,” Opt. Lett., vol.29, pp.2222-2224, 2004. The content of this article is incorporatedherein by reference.

Another scheme, requiring only a power detector, may be used tointerrogate a selected cladding mode resonance independently oftemperature or strain. This involves a pair of gratings as shown in FIG.6. The TFBG is placed at the sensing point and is followed by a highreflectivity regular FBG of the same length (hence having the samespectral bandwidth) but with a Bragg wavelength AB centered near anyoneof the cladding mode resonances (referred to as λC). Incident broadbandlight goes through the TFBG and gets attenuated at λC, continues to theFBG, gets reflected and returns towards after having passed a secondtime through the attenuating TFBG. This reflected light is recuperatedfrom the fiber using either a circulator or a 3 dB coupler. The spectralbandwidth of the input light needs only to be large enough to cover themaximum wavelength excursion of the sensor in operation. The totalamount of light reaching the detector consists of the residual Braggreflection from the TFBG (which can be minimized for certain tiltangles) and the light reflected at λB. When λC=λB, very little lightreaches the detector but as soon as the cladding mode resonance shiftsor changes its strength the power level at the detector changes. As anexample, such a pair of gratings were fabricated with λC>λB initiallyand then the FBG was stretched (thereby increasing λB) through thecladding mode resonance. Light from a pumped erbium-doped fiberbroadband source was launched into the fiber containing the two gratingsthrough a 3 dB coupler. The reflected power detected near the input ofthe fiber as a function of strain on the FBG is shown in FIG. 7. Thegraph shows that the power changes by more than 10 dB when λB becomesequal to λC. This represents a simple scheme to detect changes in Braggwavelength or coupling strength without the need for an optical spectrumanalyser. As shown in FIG. 4, if the two gratings illustrated in FIG. 6are at the same temperature then λC and λB will move together and nopower level change will be observed in the reflected signal. In mostpractical applications it would be the FBG that would provide thereference and the power level fluctuations would reflect changes in theTFBG cladding modes due to external perturbations. However the scheme issymmetrical with respect to the two gratings and the experimentalresults discussed here show that the sensitivity can be quitesignificant (0.1 dB/μStrain).

In a possible variant, the TFBG is designed to produce plasmonresonances to perform measurements on physical manifestations. A typicalsetup is shown in FIG. 9. The TFBG 900 is fabricated in an optical fiber902. The cladding 904 of the optical fiber 902 is coated with a suitablemetal layer 906, such as gold to produce a dielectric/conductorinterface 910. Such dielectric/conductor interface 910 gives rise tosurface plasmon resonances. Specifically, in order for plasmonresonances to occur, the optical field within the fiber 802 has to havenon zero amplitude at the dielectric/conductor interface 910 and aspecific value of axial propagation constant (or, if one considers theray optics approach to wave guiding, of angle of incidence within thetotal internal reflection regime).

There are many potential applications for such structures, especially inthe optical sensing of chemicals, among others. The TFBG 900 is used tocouple the core mode light to a multitude of cladding modes, dependingon the light wavelength, as shown in FIG. 8. The cladding modes havenon-zero evanescent fields extending outside the cladding diameter andhence into the metal layer 906. When the axial component of thepropagation constant of the cladding mode equals that of an SPR wave,coupling to that SPR wave can occur. A single TFBG is sufficient togenerate a wavelength dependent set of cladding modes that are“interrogating” the metal layer 906 at various angles of incidence. Thisis most easily seen from the phenomenological representation shown inFIG. 9. FIG. 9 illustrates the ray optic analogy of the coupling from aguided core mode to several cladding modes through a TFBG. Each of themodes can be individually “addressed” simply by changing the wavelengthof the guided light, and each mode strikes the cladding boundary at adifferent angle of incidence. On the other hand, FIG. 10 shows thetraditional attenuated total reflection method (usually referred to asthe Kretschmann configuration) to excite and detect SPR waves bychanging the angle of incidence of the light beam incident on the metalfilm.

If there are non-radiative SPR waves that can be guided by the metalfilm surrounded on one side by silica glass and on the other side by asuitable medium, and if these SPR waves have an effective index (alongthe axis of the fiber) that is phase matched to one of the claddingmodes effective indices, then coupling can occur between this claddingmode and the SPR wave. When this occurs, the cladding modes involvedwill experience more loss than their neighbours. The effective index ofthe i^(th) cladding mode (n^(i) _(clad)), can be calculated from theresonance position λ^(i) _(clad) by the following expression:

λ^(i) _(clad)=(n ^(i) _(eff) n ^(i) _(clad))Λ/(cos θ)   (1)

where n^(i) _(eff) is the effective index of the core mode at λ^(i)_(clad), and Λ and θ are the period and the internal tilt angle of theTFBG. The wavelengths of the cladding mode resonances that are perturbedas a result of a coupling to a SPR wave in a metal-coated TFBG, such asthe TFBG 900, the provide a direct measure of the effective index of theSPW through Equation (1).

In the course of experimental work fiber gratings were fabricated usingthe standard process of KrF excimer laser irradiation of hydrogen-loadedCORNING SMF28 fiber through a phase mask. The required tilt was achievedby rotating the mask-fiber assembly around an axis perpendicular to thefiber axis and to the plane of incidence of the laser light. Thetransmission spectrum of the grating used for the experiments reportedis shown in FIG. 8. The longest wavelength resonance corresponds to thereflection of the core mode light onto itself (Bragg wavelength), whileall the shorter wavelength resonances correspond to the excitation ofbackward propagating cladding modes. These modes are not reflected backto the source because they are rapidly attenuated by the fiber jacket assoon as they leave the grating region (where the jacket has been removedprior to fabricating the grating). For resonances between 1520 and 1560nm, phase mask periods of the order of 1 μm are used. After fabrication,the gratings were heat-stabilized by subjecting them to a rapidannealing at −300° C. and the remaining hydrogen removed by 12 hours ofheating at 120° C. prior to gold deposition. In these preliminaryexperiments, we used a small-scale sputtering chamber (PolaronInstruments model E5100) with the fiber positioned a few cm from thegold target. For flat samples in the same geometry, a gold layerthickness of 20 nm requires 1 minute of deposition at a pressure of 0.1Torr, a potential difference of 2.5 kV, and 18-20 mA of sputter current.In order to coat the fiber as uniformly as possible, two coating runswere made with the fiber holder rotated by 180 degrees between thecoatings. Under these conditions, the film uniformity around the fibercircumference is unlikely to be optimal. The film thickness on the fiberthat is indicated in this specification is the value expected for thetwo sides of the fiber that directly facing the sputtering target duringthe two coating runs. While thicknesses ranging from 10 to 50 nm weretested, the following description will focus on results obtained with a20 nm-thick nominal gold layer.

After the gold deposition, the fiber transmission spectrum is visiblymodified, but without measurable features of interest, as shown in FIG.11, indicating that the very thin gold layer has had an effect. However,no narrowband SPR resonances can be seen in the wavelength spectrumsampled by the cladding modes. When the gold-coated grating is immersedin liquids with various refractive indices (sugar solutions), anomalousresonances appear for certain very specific sugar concentrations, asdetermined from Abbe refractometer measurements of the refractive indexof the solutions at 589 nm (nD) (FIG. 12). The accuracy of the Abberefractometer that was used is of ±0.0002. These resonances are not thesame as those obtained for uncoated tilted fiber gratings, since theyhave a finite bandwidth within the cladding mode envelope and theirmaximum attenuation shifts rapidly with the external index. The peakposition of the anomalous resonance (λ_(P)) is obtained by fitting theenvelope of the cladding mode resonances. FIG. 13 shows how λ_(P)changes as the refractive index of the outer medium is increased bysmall amounts. The spatial width of the envelope of the anomalousresonances is about 5 nm.

By using equation (1) to find the effective indices of the claddingmodes within a resonance and the refractive index of silica near 1550 nm(n=1.444), it is possible to calculate the angular spread of theequivalent angles of incidences (since the effective index is equal tothe projection on the fiber axis of the refractive index in silica). Forthe data of FIG. 12, the angular spread is 3.5 degrees (around a meanincidence angle θ=78°. This angle of incidence agrees with the predictedvalue for gold-coated silica glass in sucrose solutions interrogated atwavelengths close to 1500 nm. The angular spread of the resonance alsocorresponds well to typical values obtained for SPR measurements madewith the Kretschmann configuration. Furthermore, the wavelength shift asa function of n_(D) is well approximated by a straight line with a slopeof 454 pm/(10⁻³ change in n_(D)). Even considering the dispersion of thesugar solutions between 589 nm and the 1520-1560 nm region, this isagain in quantitative agreement with the expected behavior forcontra-directional gratings in gold-coated silica fibers where shifts ofthe order of 100-500 pm/(10⁻³ change in n_(ext)) were theoreticallypredicted. These observations support the hypothesis that the resonanceseen is indeed due to a SPR that is perturbing some of the claddingmodes. In particular, the effective indices of the plasmons that areobserved are smaller than the glass refractive index but corresponds toa situation where the plasmons are seen as perturbed cladding modes witha local electromagnetic field maximum at the outer metal boundary. It isthis local field maximum that enhances the sensitivity of the claddingmode resonance to the exact value of external index.

The SPR waves can be used for chemical and biological monitoring throughchanges in the refractive index of the medium in which the fiber islocated or through changes in the refractive index of the gold layeritself.

The tilted grating discussed above can be used for a number of differentsensing applications, examples of which are discussed below:

1. Strain Gage with Thermal Compensation

As discussed earlier, the response of the tilted grating includes a coremode resonance component and a cladding mode resonance component. Thecore mode resonance component conveys the response of the sensor totemperature and elongation strain. In a specific example as thetemperature of the grating changes or as elongation strain acts on thetilted grating the peak wavelength of the core mode resonance willshift. On the other hand, the wavelength gap between the peak wavelengthof the core mode resonance and anyone of the peaks in the cladding moderesonance is generally constant with temperature. This means that as thetemperature changes this gap will also change. However, if onlyelongation strain is applied on the sensor and the temperature ismaintained constant the peaks of the core mode resonance component andof the cladding mode resonance components will shift in unisonmaintaining the gap constant. So, the gap change is indicative of thetemperature variation only. Once the wavelength shift due only totemperature is determined, it suffices to subtract this wavelength shiftfrom say the total wavelength shift of the main peak of the core moderesonance to determine the wavelength shift due only to elongationstrain. Once this wavelength shift is known, the elongation strain canbe derived easily.

FIG. 14 shows a measurement apparatus 1000 using a titled grating. Themeasurement apparatus 1000 can be used to measure elongation strain,independent of temperature variations. The measurement is performed in asensing zone 1002. Generally, the measurement apparatus 1000 has anoptical sensor 1004 which is located in the sensing zone 1002 andincludes a tilted grating, a signal processing device 1006 whichperforms an analysis of the optical response generated by the opticalsensor 1004, and an optical excitation generator 1008 that injects intothe optical sensor 1004 an optical excitation.

The sensing zone 1002 is the area where the measurement is to be made.The optical sensor 1004 has a continuous length of optical fiber. Theoptical fiber has a core in which is formed a TFBG.

In use, the optical excitation generator 1008 generates light which isinjected into the optical fiber length that leads to the optical sensor1004. The optical excitation reaches the TFBG which filters out from theoptical excitation wavelengths corresponding to the peaks in the coreand cladding mode resonances. The optical excitation that reaches thesignal processing device 1006 is lacking the wavelengths filtered out bythe TFBG. The signal processing device 1006 uses the information itreceives from the optical sensor 1004 to derive the intensity of theelongation strain acting on the optical sensor 1004 in the sensing zone1002, corrected for temperature variations in the sensing zone 1002. Ifdesired to measure pressure or displacement acting on the optical sensor1004, there may be a necessity to mount the optical sensor 1004 on atransducer structure (not shown in the drawings) that is directlyexposed to pressure or displacement and communicates this pressure ordisplacement directly to the optical sensor 1004 in the form ofelongation strain. Such transducer structures are known in the art anddo not need to be discussed here in greater details.

Note that different measurement apparatus architectures can be used. Avariant is shown in FIG. 18, where the excitation and the collection ofthe TFBG response are made from the same side of the optical fiber.Specifically, in this embodiment, light generated by the opticalgenerator 1008 is injected into the optical fiber length that leads toan optical coupler and to the optical sensor 1004. The opticalexcitation reaches the TFBG which filters out from the opticalexcitation wavelengths corresponding to the peaks in the core andcladding mode resonances. The optical fiber includes a reflector 1017that will reflect back toward the signal processing device/opticalexcitation generator combination, the response produced by the TFBG. Thereflector 1017 can be formed at a termination point of the opticalfiber. In a specific example of implementation, the termination is astraight termination. Different mechanisms can be used to providereflectivity at the reflector 1017. In one example, the reflectivity isachieved via Fresnel reflection. In a different example, thereflectivity is provided by a reflective coating deposited on thestraight termination. In another possible example, the reflector 1017 isa non-tilted grating that has a reflection spectrum wide enough tooverlap two or more of the cladding mode resonances produced by thetilted grating.

Another possibility is to form on the sensor a coating which expands inresponse to a certain substance or condition. The expansion causeselongation strain which can then indirectly either detect the substanceor measure its concentration. The specification provides below examplesof substances that respond to physical manifestations and that caninduce a measurable strain on the sensor.

The signal processing device extracts from the response from the opticalsensor 1004 information on the elongation strain acting on the opticalsensor 1004 along with the temperature in the sensing zone 1002. FIG. 15is a block diagram of the signal processing device 1006. The signalprocessing device 1006 is based on a computer platform that enables toperform digital signal processing on the response received from theoptical sensor 1004 such as to derive the information desired. Morespecifically, the signal processing device 1006 includes in inputinterface 1010 that is coupled to the optical fiber length leadingdirectly to the optical sensor 1004. The input interface 1010 willconvert the signal into an electric digital signal, including performingappropriate filtering. The digital signal is then impressed on the databus 1012 that establishes a communication path between a processor 1016and a memory device 1014. The processor 1016 executes program code thatprocesses the data in the digital signal to extract information on theelongation strain and temperature, according to the logic discussedabove.

The signal processing device 1006 also has an output interface 1018 thatallows communicating the result of the mathematical processing to anexternal entity. The external entity can be a human operator or a pieceof equipment that uses the information generated by the signalprocessing device 1006 for specific purposes.

Accordingly, the measurement apparatus 1000 can measure the elongationstrain acting on the optical sensor 1004 and the temperature in thesensing zone 1002.

2. Multi-purpose Sensor

As discussed previously, the TBFG can be used to sense two or morephysical manifestations. A senor can be provided to measure the index ofrefraction adjacent the sensing surface of the sensor, and anotherphysical manifestation such as elongation strain and/or temperature. Themeasurement of the elongation strain and/or temperature was discussedearlier. The measurement of the index of refraction can be made in twodifferent ways. One is to track a wavelength shift of the cladding moderesonances, as discussed in connection with FIGS. 2 a and 2 b. The otheris to detect the occurrence of surface plasmon resonances, bydetermining which one of the cladding modes will experience loss, asdiscussed earlier.

3. Bending and Strain Gage and/or Temperature Sensor

As discussed earlier, the cladding mode resonances of the TFBG can beused to detect bending, in particular the low order cladding modes.Accordingly, by interrogating the TFBG in the wavelength thatcorresponds to those low order cladding modes, the degree of bending ofthe sensor can be determined. In a specific example, the power ratio ofthe first two resonances in the cladding mode varies with the degree ofbending and can be used to accurately measure this parameter. At thesame time the elongation strain and/or temperature can be measured bythe techniques discussed earlier. The measurement of the degree ofbending of the sensor can be used as an indirect measure of anotherphysical manifestation which acts on the sensor to induce bending in it.For example, the sensor can be mounted on a diaphragm that is exposed topressure. The degree of pressure determines the extent to which thediaphragm bows out. A sensor placed on the diaphragm will be caused tobend accordingly, and by measuring the degree of bending one candetermine the extent to which the diaphragm bows and consequently thepressure acting on the diaphragm. This is shown in FIG. 16. Thediaphragm 2000 that is made of flexible material is exposed to pressureon its side 2002. A sensor 2004 is mounted to the diaphragm. The sensoris placed on the convex side of the diaphragm 2000 but it can also beplaced on the concave side as well.

Another possibility is to coat the sensor with a substance that inducesbending in the sensor in response to a certain physical manifestation.For example, the coating can be placed only on one side of the sensor.The material of the coating is selected such that it swells when incontact with the substance to detect. When the coating swells it inducesbending in the sensor which can be detected as indicated earlier. Byproperly selecting the coating the sensor can thus be made responsive toa wide variety of substances, such as humidity (water), chemicalsubstances and biological substances.

In one example, the substances that would be responsive to water are,but not limited to, water-swellable materials.

The water-swellable material relates, for example, to a particulateabsorbent material comprising a particulate core of absorbent polymers,coated with a coating agent, comprising or being an organic coatingcompound, which has one or more polar groups.

The absorbent polymer refers, for example, to a polymer, which iswater-insoluble, water-swellable or gelling. These polymers aretypically lightly cross-linked polymers, which contain a multiplicity ofacid functional groups such as carboxylic acid groups. Examples of acidpolymers suitable for use herein include those which are prepared frompolymerizable, acid-containing monomers, or monomers containingfunctional groups which can be converted to acid groups afterpolymerization. Thus, such monomers include olefinically unsaturatedcarboxylic acids and anhydrides, and mixtures thereof. The acid polymerscan also comprise polymers that are not prepared from olefinicallyunsaturated monomers.

Examples of such polymers include, but are not limited to,polysaccharide-based polymers such as carboxymethyl starch andcarboxymethyl cellulose, and poly(amino acid) based polymers such aspoly(aspartic acid).

Some non-acid monomers can also be included, usually in minor amounts,in preparing the absorbent polymers herein. Such non-acid monomers caninclude, for example, but not limited to, monomers containing thefollowing types of functional groups: carboxylate or sulfonate esters,hydroxyl groups, amide-groups, amino groups, nitrile groups, quaternaryammonium salt groups, and aryl groups (e.g., phenyl groups, such asthose derived from styrene monomer). Other optional non-acid monomersinclude unsaturated hydrocarbons such as ethylene, propylene, 1-butene,butadiene, and isoprene.

Olefinically unsaturated carboxylic acid and anhydride monomers usefulherein include the acrylic acids typified by acrylic acid itself,methacrylic acid, .alpha.-chloroacrylic acid, a-cyanoacrylic acid,.beta.-methylacrylic acid (crotonic acid), .alpha.-phenylacrylic acid,.beta.-acryloxypropionic acid, sorbic acid, .alpha.-chlorosorbic acid,angelic acid, cinnamic acid, p-chlorocinnamic acid,.beta.-stearylacrylic acid, itaconic acid, citroconic acid, mesaconicacid, glutaconic acid, aconitic acid, maleic acid, fumaric acid,tricarboxyethylene, and maleic anhydride.

In one specific example, the absorbent polymers contain carboxyl groups.These polymers include hydrolyzed starch-acrylonitrile graft copolymers,partially neutralized hydrolyzed starch-acrylonitrile graft copolymers,starch-acrylic acid graft copolymers, partially neutralizedstarch-acrylic acid graft copolymers, hydrolyzed vinyl acetate-acrylicester copolymers, hydrolyzed acrylonitrile or acrylamide copolymers,slightly network cross linked polymers of any of the foregoingcopolymers, polyacrylic acid, and slightly network cross linked polymersof polyacrylic acid. These polymers can be used either solely or in theform of a mixture of two or more different polymers.

Those of skills in the art will be familiar with the process of coatingthe water-swellable material onto the sensor.

In a further example, the water-swellable material includes, but is notlimited to, clay intimately mixed with a polypropene, a polybutene or amixture of polypropene and polybutene, and a clay binding ion-exchangeor coupling agent compound, to provide a composition having anunexpected capacity for swelling upon contact with water. Thecomposition should include a clay binder that is ion-exchanged with clayplatelet cations on internal negative charge sites of the clayplatelets, or reacted with hydroxyl moieties at the clay platelet edgesto achieve unexpected water-swellability.

The water-swellable clay utilized can be, but not limited to, anywater-swellable layered material, such as a smectite clay, which willswell upon contact with water. The clay may be smectite clay, such as amontmorillonite or a bentonite clay. This clay has sodium as apredominant exchange cation. However, the clay utilized may also containother cations such as magnesium and iron.

Those skilled in the art will be familiar with the methods forpreparation of the compositions described herein.

4. Chemical/Biological Sensor

The sensor using a TFBG uses an interface responsive to the biologicalor chemical element to be detected to produce a physical manifestationthat can be measured by the sensor. The interface can be designed toimpress on the sensor a physical force which can be directly measured.An example of such interface was mentioned earlier and it wouldtypically be in the form of a coating that causes the sensor to bend orstretch when it comes in contact with the biological or chemical elementto be detected.

The interface can also be such as to cause SRI changes in response tothe presence of the biological or chemical element to be detected. Thechemical or biological sensor can also function without the need of aninterface detects SRI changes which can be used in applications where adirect measure of the SRI is required or applications where the SRIchange is an indicator of the occurrence of a chemical or a biologicalprocess or element. As briefly mentioned above, the SRI can be measuredin two different manners. One involves tracking the wavelength shift byof the cladding mode resonances, as discussed in connection with FIGS. 2(a) and 2 (b). The other uses the detection of SPR.

The sensor can be used as a chemical sensor to detect changes in SRIcaused by the presence of a chemical element such that, but not limitedto, the sensor can be used for determining the concentration of sugar ina medium, such as an aqueous solution, for determining the concentrationof alcohol in a medium, for measuring the degree of curing of anadhesive, as the adhesive cures the SRI changes. By measuring the SRIone can track the degree of curing or detect a threshold at which theadhesive is considered to be cured. The invention is also used formeasuring the degree of curing of cement in a fashion similar to thecuring of an adhesive. The invention is further used as a biologicaldetector. Generally, the biological detector includes an interface.

As used herein, the term chemical or biological analyte refers to achemical or biological element to be detected.

The chemical and biological analytes that are contemplated include, butare not limited to, bacteria; yeasts; fungi; viruses; rheumatoid factor;antibodies, including, but not limited to IgG, IgM, IgA and IgEantibodies; carcinoembryonic antigen; streptococcus Group A antigen;antigen; viral antigens; antigens associated with autoimmune disease;allergens; tumor antigens; streptococcus Group B antigen, HIV I or HIVII antigen; or host response (antibodies) to these and other viruses;antigens specific to RSV or host response (antibodies) to the virus; anantibody; antigen; enzyme; hormone; polysaccharide; protein; prions;lipid; carbohydrate; drug; nucleic acid; Salmonella species; Candidaspecies, including, but not limited to Candida albicans and Candidatropicalis; Salmonella species; Neisseria meningitides groups A, B, C, Yand W sub 135, Streptococcus pneumoniae; E. coli K1. E. coli;Haemophilus influenza type B; an antigen derived from microorganisms; ahapten; a drug of abuse; a therapeutic drug; environmental agents; andantigens specific to Hepatitis; an enzyme; a DNA fragment; an intactgene; a RNA fragment; a small molecule; a metal; a toxin; a nucleicacid; a cytoplasm component; pili or flagella component; or any otheranalytes.

The analyte of interest would typically be in a carrier medium which canbe solid, gel-like, liquid or gas. For instance analyte can be detectedin a bodily fluid such as mucous, saliva, urine, fecal analyte, tissue,marrow, cerebral spinal fluid, serum, plasma, whole blood, sputum,buffered solutions, extracted solutions, semen, uro-genital secretion,pericardial, gastric, peritoneal, pleural, throat swab, subfractionsthereof, or other washes and the like. A gaseaous medum may be, but notlimited to, air.

An aqueous buffered solution may be employed to dilute the analyte, suchan aqueous buffer solution may be lightly or heavily buffered dependingon the nature of the analyte to be detected. Various buffers may beemployed such as carbonate, phosphate, borate, Tris, acetate, barbital,Hepes, or the like. Organic polar solvents, e.g., oxygenated neutralsolvents, may be present in amounts ranging from about 0 to 40 volumepercent such as methanol, ethanol, .alpha.-propanol, acetone,diethylether, or the like.

In one specific example, the interface has a recognition analyte thatcan be attached on the sensing surface of the sensor and will associatewith the analyte of interest (FIG. 1). When such binding occurs the SRIchanges. The change is specific in that the SRI acquires a specificvalue which indicates that the binding has taken place. The claddingmode resonances “interrogate” the electric/dielectric interface and whenthe SRI acquires the specific value, the energy in at least one of theresonance will be transferred into a SPR. The energy transfer will showas a reduced power in that particular cladding mode resonance, allowingdetermining that the SRI is at the specific value indicative of abinding event. Note that since several resonances interrogate theelectric/dielectric interface, this technique can at least in theorymonitor simultaneously for a set of different specific SRI values, eachassociated to a given resonance in the cladding mode resonancecomponent. As such, several binding events, corresponding to differentSRI values, can be detected simultaneously.

The recognition analyte is thus a component of a specific binding pairand includes, but is not limited to, antigen/antibody, enzyme/substrate,oligonucleotide/DNA, chelator/metal, enzyme/inhibitor,bacteria/receptor, virus/receptor, hormone/receptor, DNA/RNA, orRNA/RNA, oligonucleotide/RNA, and binding of these species to any otherspecies, as well as the interaction of these species with inorganicspecies.

The recognition analyte that is attached to the sensing surface can be,but is not limited to, toxins, antibodies, antigens, hormone receptors,parasites, cells, haptens, metabolizers, allergens, nucleic acids,nuclear analytes, autoantibodies, blood proteins, cellular debris,enzymes, tissue proteins, enzyme substrates, coenzymes, neurontransmitters, viruses, viral particles, microorganisms, proteins,polysaccharides, chelators, drugs, and any other member of a specificbinding pair. The recognition analyte is specifically designed toassociate with the analyte of interest.

The recognition analyte may be passively adhered to the sensing surface.If required, the recognition analyte may be covalently attached to thesensing surface of the sensor. The chemistry for attachment ofrecognition analyte is well known to those skilled in the art.

Recognition analyte for detection of bacteria may have binding activityto specifically bind a surface membrane component, protein or lipid, apolysaccharide, a nucleic acid, or an enzyme. The analyte, which isspecific to the bacteria, may be a polysaccharide, an enzyme, a nucleicacid, a membrane component, or an antibody produced by the host inresponse to the bacteria. The presence of the analyte may indicate aninfectious disease (bacterial or viral), cancer or other metabolicdisorder or condition. The presence of the analyte may be an indicationof food poisoning or other toxic exposure. The presence of the analytemay also be an indication of bacterial contamination.

A wide range of techniques can be used to apply the recognition analyteof the interface to the sensing surface of the sensor. The sensingsurface may be, for example but not limited to, coated with recognitionanalyte by total immersion in a solution for a predetermined period oftime; application of solution in discrete arrays or patterns; spraying,ink jet, or other imprinting methods; or by spin coating from anappropriate solvent system. The technique selected should minimize theamount of recognition analyte required for coating a large number oftest surfaces and maintain the stability/functionality of recognitionanalyte during application.

Attachment of the Recognition Analyte By Self-Assembled Monolayers onSensing Surface

In a further embodiment, the invention includes attachment of therecognition analyte into the surface or the interior of the sensingsurface, through self-assembled monolayers.

Self-assembled monolayers can be prepared using different types ofmolecules and different substrates. Commonly used examples are, but notlimited to, alkylsiloxane monolayers, fatty acids on oxidic materialsand alkanethiolate monolayers. This type of self-assembled monolayersholds great promise for applications in several different areas. Someexamples of suggested and implemented applications are, but not limitedto, molecular recognition, self-assembly monolayers as model substratesand biomembrane mimetics in studies of biomolecules at surfaces,selective binding of enzymes to surfaces.

There are many different systems of self-assembling monolayers based ondifferent organic components and supports, such as, but not limited to,systems of alkanethiolates, HS(CH.sub.2).sub.n R, on gold layers. Thealkanethiols chemisorb on the gold surface from a solution in which thegold layer is immersed, and form adsorbed alkanethiolates with loss ofhydrogen. Adsorption can also occur from the vapor. Self-assemblingmonolayers formed on gold from long-chain alkanethiolates of structurex(CH.sub.2).sub.n Y(CH.sub.2).sub.m S are highly ordered. A wide varietyof organic functional groups (X,Y) can be incorporated into the surfaceor interior of the monolayer.

In one example, the self-assembling monolayer is formed of acarboxy-terminated alkane thiol stamped with a patterned elastomericstamp onto a gold surface. The alkanethiol is inked with a solution ofalkanethiol in ethanol, dried, and brought into contact with a surfaceof gold. The alkanethiol is transferred to the surface only at thoseregions where the stamp contacts the surface, producing a pattern ofself-assembling monolayer which is defined by the pattern of the stamp.Optionally, areas of unmodified gold surface next to the stamped areascan be rendered hydrophobic by reaction with a methyl-terminated alkanethiol. The details of the method are well known in the art.

The present invention, the self-assembling monolayer has the followinggeneral formula: X is reactive with metal or metal oxide. For example, Xmay be asymmetrical or symmetrical disulfide (—R′SSY′, —RSSY), sulfide(—R′SY′, —RSY), diselenide (—R′Se—SeY'), selenide (—R′SeY′, —RSeY),thiol (—SH), nitrile (—CN), isonitrile, nitro (—NO.sub.2), selenol(—SeH), trivalent phosphorous compounds, isothiocyanate, xanthate,thiocarbamate, phosphine, thioacid or dithioacid, carboxylic acids,hydroxylic acids, and hydroxamic acids.

R and R′ are hydrocarbon chains which may optionally be interrupted byhetero atoms and which are preferably non-branched for the sake ofoptimum dense packing. At room temperature, R is greater than or equalto seven carbon atoms in length, in order to overcome naturalrandomizing of the self-assembling monolayer. At colder temperatures, Rmay be shorter. In one embodiment, R is —(CH.sub.2).sub.n—where n isbetween 10 and 12, inclusive. The carbon chain may optionally beperfluorinated. A person skilled in the art will understand that thecarbon chain may be of any length.

Y and Y′ may have any surface property of interest. For example, Y andY′ could be any among the great number of groups used for immobilizationin liquid chromatography techniques, such as hydroxy, carboxyl, amino,aldehyde, hydrazide, carbonyl, epoxy, or vinyl groups.

Self-assembling monolayers of alkyl phosphonic, hydroxamic, andcarboxylic acids may also be useful for the methods of the presentinvention. Since alkanethiols do not adsorb to the surfaces of manymetal oxides, carboxylic acids, phosphonic acids, and hydroxamic acidsmay be preferred for X for those metal oxides.

R may also be of the form (CH.sub.2).sub.a —Z—(CH.sub.2).sub.b, wherea.gtoreq.0, B.gtoreq.7, and Z is any chemical functionality of interest,such as sulfones, urea, lactam, and the like.

The stamp may be applied in air, gel, semi-gel, or under a fluid, suchas water to prevent excess diffusion of the alkanethiol,. Forlarge-scale or continuous printing processes, it is most desirable toprint in air, since shorter contact times are desirable for thoseprocesses.

In one specific example the sensor can be used in immunoassay methodsfor either antigen or antibody detection. The sensors may be adapted foruse in direct, indirect, or competitive detection schemes, fordetermination of enzymatic activity, and for detection of small organicmolecules such as, but not limited to drugs of abuse, therapeutic drugs,environmental agents), as well as detection of nucleic acids andmicroorganisms.

For immunoassays, an antibody may serve as the recognition analyte or itmay be the analyte of interest. The recognition analyte, for example anantibody or an antigen, should form a stable, dense, reactive layer onthe attachment layer of the test sensor. If an antigen is to be detectedand an antibody is the recognition analyte, the antibody should bespecific to the antigen of interest; and the antibody (recognitionanalyte) should bind the antigen (analyte) with sufficient avidity thatthe antigen is retained at the surface of the sensing surface. In somecases, the analyte may not simply bind the recognition analyte, but maycause a detectable modification of the recognition analyte to occur.This interaction could cause an increase in mass at the test surface ora decrease in the amount of recognition analyte on the test surface. Anexample of the latter is the interaction of a degradative enzyme oranalyte with a specific, immobilized substrate. In this case, one wouldsee a diffraction pattern before interaction with the analyte ofinterest, but the diffraction pattern would disappear if the analytewere present.

In another specific example, the sensor applies to detection of nucleicacid molecules and nucleic acid probes. Nucleic acids can be attached tosensing surfaces in an hybridization assays. A sensing surface such asgold is modified with nucleic acids via for example, but not limited to,linkers, and blocking moieties, which serve to shield the nucleic acidsfrom the sensing surface.

“Blocking moieties” are molecules which are attached to the sensingsolid support and function to shield the nucleic acids from the sensingsurface. For the purposes of this invention, the attachment of a sulfurmoiety to a sensing surface, such as gold, is considered covalent.

By “nucleic acids” or “oligonucleotides” herein is meant at least twonucleotides covalently linked together. A nucleic acid of the presentinvention will generally contain phosphodiester bonds, although in somecases, as outlined below, a nucleic acid analogs are included that mayhave alternate backbones, comprising, for example, phosphoramide,phosphorothioate, phosphorodithioate, O-methylphophoroamidite linkagesand peptide nucleic acid backbones and linkages.

The nucleic acids of the invention may also be characterized as “probe”nucleic acids and “target” nucleic acids. These terms are known in theart. Either probe or target nucleic acids may be attached to the solidsupport via linker. In a preferred embodiment, the probe nucleic acidsare attached, via linker moieties, to the solid support, and the targetnucleic acids are added in solution. The nucleic acid and the probe maybe labeled.

Probe nucleic acids or probe sequences are preferably single strandednucleic acids. The probes of the present invention are designed to becomplementary to the target sequence, such that hybridization of thetarget sequence and the probes of the present invention occur. Thiscomplementarity need not be perfect; there may be any number of basepair mismatches which will interfere with hybridization between thetarget sequence and the single stranded probe nucleic acids of thepresent invention. However, if the number of mutations is so great thatno hybridization can occur under even the least stringent ofhybridization conditions, the sequence is not a complementary targetsequence.

It will be appreciated by those in the art, the length of the probe willvary with the length of the target sequence and the hybridization andwash conditions.

In a possible variant, the nucleic acid is attached to the sensingsurface in monolayers. The techniques to attach nucleic acid moleculesto the sensing surface will be well known to those skilled in the art.

“Target nucleic acids” or “sequences” means a nucleic acid sequence on asingle strand of nucleic acid. The target sequence may be a portion of agene, a regulatory sequence, genomic DNA, cDNA, mRNA, or others. It maybe any length, with the understanding that longer sequences are morespecific. As is outlined herein, probes are made to hybridize to targetsequences to determine the presence or absence of the target sequence ina sample. Target nucleic acids may be prepared or amplified as iscommonly known in the art. When target nucleic acids are attached to thesensing surface, they will generally be the same size as outlined forprobe nucleic acids, above.

In general, blocking moieties have at least a first and a second end.The first end is used to covalently attach the blocking moiety to thesensing surface. The second end terminates in a terminal group, definedbelow. However, in some embodiments, the blocking moieties may bebranched molecules. Thus, for example, the first end is used forattachment to the solid support and all or some of the other ends mayterminate in a terminal group, as defined below.

The second end of the blocking moiety terminates in a terminal group. By“terminal group” or “terminal moiety” herein is meant a chemical groupat the terminus of the blocking moiety. The terminal groups may bechosen to modulate the interaction between the nucleic acid and theblocking moieties, or the surface. Thus, for example, in anotherembodiment, when the blocking moieties form a monolayer as is generallydescribed below, the terminal group may be used to influence the exposedsurface of the monolayer. Thus, for example, the terminal group may beneutral, charged, or sterically bulky. For example, the terminal groupsmay be negatively charged groups, effectively forming a negativelycharged surface such that when the probe or target nucleic acid is DNAor RNA the nucleic acid is repelled or prevented from lying down on thesurface, to facilitate hybridization. This may be particularly usefulwhen the nucleic acid attached to the sensing surface via a linkermoiety is long.

In addition to the blocking moieties, the sensing surface of theinvention comprises modified nucleic acids.

The nucleic acids of the invention are modified with linker moieties, toform modified nucleic acids which are attached to the sensing surface.By “modified nucleic acid” herein is meant a nucleic acid as definedabove covalently attached to a linker moiety.

By “linker moieties” is meant molecules which serve to immobilize thenucleic acid at a distance from the sensing surface. Linker moietieshave a first and a second end. The first end is used to covalentlyattach the linker moiety to the sensing surface. The second end is usedfor attachment to the nucleic acid.

The blocking moieties are made using techniques well known in the art.

In a further variant, the present invention is useful in methods ofassaying for the presence or absence of target nucleic acids in thesample to be analyzed. Thus, the present invention provides methods ofhybridizing probe nucleic acids to target nucleic acids. The methodscomprise adding or contacting target nucleic acids to a sensing surfaceof the invention. The sensing surface comprises blocking moieties, andmodified probe nucleic acids. The contacting is done under conditionswhere the probe and target nucleic acids, if suitably complementary,will hybridize to form a double-stranded hybridization complex.

The assay conditions may vary, as will be appreciated by those in theart, and include high, moderate or low stringency conditions as is knownin the art. The assays may be done at a variety of temperatures, andusing a variety of incubation times, as will be appreciated by those inthe art. In addition, a variety of other reagents may be included in thehybridization assay, including buffers, salts, proteins, detergents orthe like. Positive and negative controls are generally run.

In a further specific example, the sensor is used to detectmicroorganisms in medium.

The term “microorganism” as used herein means an organism too small tobe observed with the unaided eye and includes, but is not limited tobacteria, a cell, cells, viruses, protozoans, fungi, and ciliates.

In another embodiment, microorganisms such as, but not limited to,bacteria, bacteriophages, viruses, and cellular analyte can be detectedby sampling the nucleic acids, such as, but not limited to, nucleotidesor polynucleotides that they contain or release. Microorganisms can alsobe detected by sampling the protein, carbohydrates and/or lipids thatthey contain or release.

In a possible variant, the microorganims may be detected by arecognition analyte such as not limited to an antibody assembled as amonolayer on the sensing surface or may be detected by interactingdirectly on the bare sensing surface.

The embodiments of the invention described herein can be used in severalapplication areas, for example, but not limited to, for the quantitativeor qualitative determination of chemical, biochemical or biologicalanalytes in screening assays in pharmacological research, for real-timebinding studies or in the determination of kinetic parameters inaffinity screening or in research, for DNA and RNA analytics and for thedetermination of genomic or proteomic differences in the genome, for thedetermination of protein-DNA interactions, for the determination ofregulation mechanisms for mRNA expression and protein (bio)synthesis,for the determination of biological or chemical markers, such as mRNA,proteins, peptides or low molecular organic (messanger) compounds, forthe determination of antigens, pathogens or bacteria in pharmacologicalproduct research and development, for therapeutic drug selection, forthe determination of pathogens, harmful compounds or germs, such as, butnot lifted to, salmonella, prions, viruses and bacteria.

Although various embodiments have been illustrated, this was for thepurpose of describing, but not limiting, the invention. Variousmodifications will become apparent to those skilled in the art and arewithin the scope of this invention, which is defined more particularlyby the attached claims.

1-38. (canceled)
 39. A method for detecting the presence of a virus in a medium, comprising: a) providing a sensor having a sensing surface, said sensor having an optical pathway containing a tilted grating; b) placing the sensing surface in contact with the medium; and c) determining from a response of said sensor if the virus is present in the medium.
 40. A method for measuring the concentration of sugar in a medium, comprising: a) providing a sensor having a sensing surface, said sensor having an optical pathway containing a tilted grating; b) placing the sensing surface in contact with the medium; and c) determining from a response of said sensor the concentration of sugar in the medium.
 41. A method for measuring the concentration of alcohol in a medium, comprising: a) providing a sensor having a sensing surface, said sensor having an optical pathway containing a tilted grating; b) placing the sensing surface in contact with the medium; and c) determining from a response of said sensor the concentration of alcohol in the medium.
 42. A method for measuring a degree of curing of a curable material, comprising: a) providing a sensor having a sensing surface, said sensor having an optical pathway containing a tilted grating; b) placing the sensing surface in contact with the curable material; and c) determining from a response of said sensor the degree of curing of the curable material.
 43. The method as defined in claim 42, wherein the material is an adhesive.
 44. The method as defined in claim 42, wherein the material is cement. 45-59. (canceled)
 60. An elongation strain sensor, comprising: a) an optical pathway; and b) a tilted grating in said optical pathway to generate a response conveying information on elongation strain acting on said sensor.
 61. The elongation strain sensor as defined in claim 60, wherein said optical pathway comprises an optical fiber.
 62. The elongation strain sensor as defined in claim 60, wherein the response comprises a core mode resonance component and a cladding mode resonances component.
 63. A method for measuring elongation strain, comprising: a) receiving a response from a sensor containing a tilted grating subjected to elongation strain, the response conveying information on: i) reaction of the tilted grating to elongation strain; and ii) reaction of the tilted grating to temperature; and b) processing the response of the tilted grating to distinguish the reaction of the tilted grating to elongation strain from the reaction of the tilted grating to temperature.
 64. An apparatus for measuring elongation strain, comprising: a) an elongation strain sensor, having: i) an optical pathway; and ii) a tilted grating in said optical pathway to generate a response conveying information on elongation strain and temperature acting on said sensor; and b) a signal processing unit to process the response of the tilted grating and distinguish in the response to reaction of said tilted grating to elongation strain from the reaction of the tilted grating to temperature.
 65. The apparatus as defined in claim 64, wherein said optical pathway comprises an optical fiber.
 66. The apparatus as defined in claim in claim 64, wherein the response of said tilted grating comprises a core mode resonance component and a cladding mode resonances component.
 67. A bending strain sensor, comprising: a) an optical pathway; and b) a tilted grating in said optical pathway to generate a response conveying information on bending strain acting on said sensor.
 68. The bending strain sensor as defined in claim 67, wherein said optical pathway comprises an optical fiber.
 69. The bending strain sensor as defined in claim 67, wherein the response comprises a core mode resonance component and a cladding mode resonances component.
 70. A method for measuring bending strain, comprising: a) receiving a response from a sensor containing a tilted grating subjected to bending strain, the response conveying information on: i) reaction of the tilted grating to bending strain; and ii) reaction of the tilted grating to temperature; and b) processing the response of the tilted grating to distinguish the reaction of the tilted grating to bending strain from the reaction of the tilted grating to temperature.
 71. An apparatus for measuring bending strain, comprising: a) a bending strain sensor, having: i) an optical pathway; and ii) a tilted grating in said optical pathway to generate a response conveying information on bending strain and temperature acting on said tilted grating; and b) a signal processing unit to process the response of the tilted grating and distinguish in the response to reaction of said tilted grating to bending strain from the reaction of the tilted grating to temperature.
 72. The apparatus as defined in claim 71, wherein said optical pathway comprises an optical fiber.
 73. The apparatus as defined in claim in claim 71, wherein the response of said tilted grating comprises a core mode resonance component and a cladding mode resonances component. 74-81. (canceled)
 82. A pressure sensor, comprising: a) an optical pathway; b) a tilted grating in said optical pathway to generate a response conveying information on bending strain acting on said sensor; and c) a flexible member, said optical pathway being mounted to said flexible member, said flexible member flexing in response to pressurized fluid and inducing in said tilted grating bending strain.
 83. The pressure sensor as defined in claim 82, wherein said optical pathway comprises an optical fiber.
 84. The pressure sensor as defined in claim in claim 82, wherein the response of said tilted grating comprises a core mode resonance component and a cladding mode resonances component. 85-101. (canceled)
 102. A method for detecting the presence of a chemical or biological element in a medium, comprising: a) providing a sensor having a sensing surface, said sensor having an optical pathway containing a tilted grating; b) placing the sensing surface in contact with the medium; and c) determining from a response of said sensor if the chemical or biological element is present in the medium.
 103. The method as defined in claim 102, wherein the chemical or biological element is a microorganism.
 104. The method as defined in claim 102, wherein the chemical or biological element is a nucleic acid. 105-107. (canceled) 